1. Field of the Invention
This invention relates to the recovery of petroleum from underground reservoirs and pertains in particular to the breaking of emulsions of crude oil, water, and surfactant that are recovered from a producing well of the reservoir.
2. Description of the Prior Art
The petroleum industry has recognized for many years that only a small fraction of the original oil in place in a reservoir is expelled by natural mechanisms. It is also well known that conventional methods of supplementing natural recovery are relatively inefficient. Typically, a reservoir may retain half of its original oil, even after the application of currently available methods of secondary recovery. Accordingly, there is a continuing need for improved recovery methods that will substantially increase the ultimate yield of petroleum from subterranean reservoirs.
Waterflooding is by far the most economical and widely practiced of secondary recovery methods. In such a process, water is injected through an input well to drive oil from the formation into an offset production well. Most of the current work in secondary recovery technology has been directed toward improving the efficiency of the waterflooding processes.
Surface active agents, or surfactants, are one class of materials that have been proposed for improving the efficiency of waterflooding processes. Much of the oil that is retained in the reservoir after a typical waterflood is in the form of discontinuous globules or discrete droplets that are trapped within the pore spaces of the reservoir. Because the normal interfacial tension between the reservoir oil and water is so high, these discrete droplets are unable to sufficiently deform to pass through narrow constrictures in the pore channels. When surfactants are added to the flooding water, they lower the interfacial tension between the water and the reservoir oil and permit the oil droplets to deform and flow with the floodwater.
In more recent years, a special surfactant flooding medium has developed, namely "microemulsions". A "microemulsion" is defined as a stable, transparent or translucent micellar solution of oil, water, and one or more surfactants. The solution may optionally contain co-surfactants and/or co-solvents.
In those reservoirs that have been subjected to a surfactant-based flood, the recovered liquid from a producing well is usually in the form of an oil-water-surfactant emulsion which is quite stable because of the reduced interfacial tension. Emulsions (not to be confused with microemulsions) are mixtures in which one liquid is uniformly distributed as discrete droplets in another liquid. The petroleum industry has long sought economical and efficient methods for breaking emulsions produced from surfactant-based floods.
Many approaches have been suggested for breaking an oil-water-surfactant emulsion. For example, U.S. Pat. No. 4,029,570 issued to Coffman, et al. on June 14, 1977 teaches contacting the emulsion with formation brine to produce an oil phase and a water phase. Unfortunately, the process is slow and the produced oil phase usually contains unacceptable amounts of water. The generally accepted maximum water content of oil destined for pipeline transmission is less than 1 wt %. Hence, additional treatments to remove the water are necessary before delivering the separated oil to a pipeline.
Another process is disclosed in U.S. Pat. No. 4,261,812 issued to Newcombe on April 14, 1981, where additional brine and surfactant are added to the emulsion. The results are said to be better than in Coffman, but problems still exist. The oil is not pipeline quality, and the surfactant is lost in the oil phase.
Yet another method for breaking produced emulsions is disclosed in U.S. Pat. No. 4,277,352, issued to Allison et al., on July 7, 1981 which involves extracting surfactants from crude oil by using various solubilizers, such as alcohols or phenols. This process requires a long equilibration period which severely limits its usefulness.
Conventional methods for breaking oil-water emulsions are generally unsuccessful in breaking emulsions which contain a microemulsion as one of the components. Further, prior art methods generally do not result in an aqueous surfactant-containing phase that is suitable for reinjection into a formation.